This invention relates to a method of measuring the velocity and distance of movement of an object indirectly (or contactlessly) with high accuracy.
It is industrially essential to measure the quantity of movement of a moving object, such as a steel plate, a steel wire or an electric wire, with respect to both its velocity and distance with high accuracy. In this case, in general, the measurement must be carried out indirectly, or without contacting the object, because of the temperature or configuration of the object. In a conventional method of this type, a spatial filter is employed. For instance, as shown in FIG. 1, the optical irregularity of the surface of an object 1 which is moving at a velocity V is optically projected, for instance by reflection, through a lens 2 onto a spatial filter 4 in which slit-shaped photocells 3 are arranged in the movement direction of the object 1. The surface irregularity is converted into an electrical signal in the spatial filter 4, and the output signal of the latter is applied through lead wires 5 to a measuring means (not shown) thereby to measure the quantity of movement of the object. That is, the output signal of the spatial filter 4 is a composite current whose intensity is varied whenever the image of the moving object 1 projected onto the spatial filter 4 passes one period of the slit train of the photocells 3 and, accordingly, an output frequency proportional to the movement velocity of the object can be obtained from this output signal. If it is assumed that the optical system's magnification is 1/m, the period of the slit train of the photocells 3 is p, and the output frequency is f, then the movement velocity V of the object can be expressed by V=mpf. Furthermore, the distance of movement of the object 1 can be expressed by the time-integration value of the movement velocity V of the object 1, that is, the integration value of the output frequency f of the spatial filter 4.
In the above-described conventional method, in order to measure the quantity of movement of the object with high accuracy, it is absolutely necessary to maintain the lens system's magnification 1/m unchanged, or to maintain the distance between the moving object 1 and the lens 2 constant; however, it is very difficult to do so according to present techniques. For instance, in the case where the moving object 1 is an electric wire which is moved at a certain velocity Va as shown in FIG. 2, it is liable to vibrate vertically and horizontally, that is, the distance L between the lens 2 and the moving object 1 is often varied. Furthermore, in some manufacturing processes, it is necessary to measure the quantity of movement of an object (not shown) which is not uniform in configuration and size. In these cases, it is almost impossible to maintain the lens system's magnification constant. Thus, heretofore, it has been impossible to measure the quantity of movement of a moving object with high accuracy.